Engine start control system for an electrically variable transmission

ABSTRACT

A system and method of controlling first and second electric motors of a vehicle having an electrically variable transmission during an engine start/stop operation. The system and method determine an input speed profile and an input acceleration profile based on an optimum engine speed, determine a requested output torque based on a plurality of torque limits and a desired output torque, determine first and second feedforward motor torques based on a requested output torque and the input speed and input acceleration profiles, determine first and second feedback motor torques based on a difference between the input speed profile and an actual input speed, and using the feedforward and feedback first and second motor torques to control the operation of the first and second electric motors when an engine is being turned on or off.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/513,061, filed Jul. 29, 2011.

FIELD

The present disclosure relates to the control of an automotivetransmission, specifically to an engine start control system and methodfor a vehicle having an electrically variable transmission.

BACKGROUND

Some current hybrid electrically variable transmissions feature twoelectric motors coupled to an internal combustion engine utilizing aplurality of clutches and gear sets. At certain times it is desirable tooperate the transmissions in strictly an electric mode or in a hybridmode where the internal combustion engine and one or both motors operatesimultaneously. Managing the many parameters such as clutch, engine andmotor torques, battery power levels and usage, efficiency and smoothshifting between the various gears and drive modes, fuel economy,operational-cost efficiency, etc. pose many operational controlchallenges.

Thus, there remains a need for continuous improvement in the operationalcontrol of hybrid electrically variable transmissions.

SUMMARY

In one form, the present disclosure provides an engine start controlsystem for a vehicle having an electrically variable transmission. Thecontrol system comprises a supervisory controller adapted to inputvehicle operating conditions and driver inputs, said supervisorycontroller adapted to determine and output a plurality of torque limits,a desired output torque, and an optimum engine speed; an input speedprofiler adapted to generate and output an input speed profile and aninput acceleration profile based on the optimum engine speed; aconstraints evaluator adapted to generate and output a requested outputtorque based on the plurality of torque limits and desired outputtorque; a feedforward controller adapted to generate first and secondfeedforward motor torques based on the requested output torque and theinput speed and input acceleration profiles; and a feedback controlleradapted to generate first and second feedback motor torques based on adifference between the input speed profile and an actual input speed.The feedforward and feedback first and second motor torques are used tocontrol the operation of the first and second electric motors when theengine is being turned on or off.

The present disclosure also provides a method of controlling first andsecond electric motors of a vehicle having an electrically variabletransmission during an engine start/stop operation. The disclosed methodcomprises using a processor to perform the steps of determining an inputspeed profile and an input acceleration profile based on an optimumengine speed; determine a requested output torque based on a pluralityof torque limits and a desired output torque; determine first and secondfeedforward motor torques based on a requested output torque and theinput speed and input acceleration profiles; determine first and secondfeedback motor torques based on a difference between the input speedprofile and an actual input speed; and using the feedforward andfeedback first and second motor torques to control the operation of thefirst and second electric motors when the engine is being turned on oroff.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description, drawings and claims providedhereinafter. It should be understood that the detailed description,including disclosed embodiments and drawings, are merely exemplary innature intended for purposes of illustration only and are not intendedto limit the scope of the invention, its application or use. Thus,variations that do not depart from the gist of the invention areintended to be within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a lever diagram of a drive system for avehicle with an electrically variable transmission;

FIG. 2 is an illustration of an example drive cycle for a vehicle inaccordance with the present disclosure;

FIG. 3 is a table describing the various drive cycle points illustratedin FIG. 2;

FIG. 4 is a block diagram of a portion of a vehicle's powertrainincorporating a controller for executing the methodology disclosedherein;

FIG. 5 is a graph of engine, motor and output speeds versus time forelectric vehicle operation in accordance with the present disclosure;and

FIG. 6 is a graph of engine, motor and output torques versus time forunder and over drive modes of operation in accordance with the presentdisclosure.

DETAILED DESCRIPTION

U.S. application Ser. No. 12/882,936, (the “'936 Application) filed Sep.15, 2010 and titled “Multi-Speed Drive Unit,” discloses variouscompound-input electrically variable transmissions (“EVT”), thedisclosure incorporated herein by reference. U.S. application Ser. No.13/188,799; filed Jul. 22, 2011; titled “Clutch System for aTransmission”, the disclosure incorporated herein by reference,discloses a clutch system that can be used e.g., in the '936Application's multi-speed drive unit to create a two dry “clutch” drivesystem, similar to a DDCT (dual dry clutch transmission), for the driveunit. FIG. 1 is an example lever diagram of such a drive system 10.

A seen in FIG. 1, the drive system 10 includes a first planetary gearset represented by a first lever L₁ and a second planetary gear setrepresented by a second lever L₂. A ring gear R₁ of the first planetarygear set is connected to an internal combustion engine ICE via an inputshaft 12. A sun gear S₁ of the first planetary gear set is connected totwo clutches CB₁, C₂. In the illustrated example, the first clutch CB₁is a braking mechanism that, when activated, grounds the sun gear S₁ tothe drive unit's transaxle case. When activated, the second clutch C₂connects the sun gear S₁ to the engine. An input brake is created whenboth clutches CB₁, C₂ are activated at the same time.

The carriers of the planetary gear sets are connected via a main shaft14. A sun gear S₂ of the second planetary gear set is connected to afirst electric motor EMA. A ring gear R₂ of the second planetary gearset is connected to a second electric motor EMB via a motor speedreducer (“MSR”) 16. The ring gear R₂ of the second planetary gear set isalso connected to an output shaft 18. The motor speed reducer 16controls the speed ratio between the second electric motor EMB and theoutput shaft 18.

The '936 Application discloses three input ratios. A first ratio iscreated by activating the first clutch CB₁ while deactivating the secondclutch C₂. A second ratio is created by deactivating the first clutchCB₁ while activating the second clutch C₂. The third ratio is the inputbrake created by activating the first and second clutches CB₁, C₂. Thereis a need to control the drive system 10 to efficiently switch betweendrive modes and gear ratios to optimize the system's and vehicle'sperformance and to improve fuel economy.

FIG. 2 is an illustration of an example drive cycle for a vehiclecontaining a FIG. 1 drive system 10 and being controlled in accordancewith the present disclosure. In the example, the vehicle acceleratesfrom a stop, cruises at high speed and brakes to a stop. The variouspoints and switching points of the drive cycle are listed in the tableshown in FIG. 3.

During the cycle, the system 10 enters different modes to deliver therequired output power from the electric motors and/or engine to theoutput shaft. The modes are chosen for best fuel economy and drivequality. The system 10 will operate in the following modes: input brakeelectric vehicle (“IB-EV”), under drive electric vehicle (“UD-EV”), overdrive electric vehicle (“OD-EV”), under drive engine on (“UD-EO”), overdrive engine on (“OD-EO”), and neutral (N). As shown in the table ofFIG. 3, there are points and modes when the electric motors arepropelling the vehicle without assistance from the engine (e.g., pointA), propelling the vehicle with the assistance from the engine (e.g.,points D to E) or providing regenerative braking (e.g., point G).

Both clutches CB₁ and C₂ will be applied (i.e., engaged or activated) toimplement the IB-EV mode. The first clutch CB₁ will be applied while thesecond clutch C₂ is not applied (i.e., disengaged or deactivated) toimplement the UD-EV and UD-EO modes. The first clutch CB₁ will not beapplied while the second clutch C₂ is applied to implement the OD-EV andOD-EO modes. Both clutches CB₁ and C₂ will be disengaged in the neutralmode. It should be appreciated that this disclosure refers to the firstclutch CB₁ as a braking clutch, but the disclosure is not limited to abraking clutch; as shown in the '936 application, many clutches orsynchronizers could be used in the system 10.

The aspects of the present disclosure are designed to control thestarting and shutdown of the engine ICE at certain points (i.e., pointsC and F) during the drive cycle. The engine start control methoddescribed herein is implemented on various components illustrated inFIG. 4, which is a block diagram of a portion of a vehicle's powertrain400 constructed in accordance with the principles disclosed herein. Thepowertrain 400 incorporates a supervisory hybrid electric vehiclecontroller 402, an input speed profiler 404, a constraints evaluator406, a feedforward controller 408, feedback controller 410, the engine,first and second clutches CB₁, C₂, first and second electric motorcontrollers 412, 414 for respectively controlling the first and secondelectric motors EMA, EMB, and a hybrid transmission plant 420.

The supervisory hybrid electric vehicle controller 402 inputs pedalposition and various other vehicle operating conditions and parametersdiscussed herein. Outputs from the supervisory hybrid electric vehiclecontroller 402 are sent to the engine, clutches CB₁, C₂, input speedprofiler 404 and constraints evaluator 406. Outputs from the engine,clutches CB₁, C₂, input speed profiler 404 and constraints evaluator 406are used by the feedforward controller 408, and feedback controller 410to control the motor controllers 412, 414 and the hybrid transmissionplant 420 to perform, among other things, the engine start controlprocess described below. Example inputs, outputs and functions of thesupervisory hybrid electric vehicle controller 402 are described in U.S.application Ser. No. 61/513,080; filed Jul. 29, 2011; titled “ModeSelection Control System for an Electrically Variable Transmission”,U.S. application Ser. No. 61/513,112; filed Jul. 29, 2011; titled “MotorOperation Control System for an Electrically Variable Transmission”, andU.S. application Ser. No. 61/513,150;filed Jul. 29, 2011; titled “ShiftExecution Control System for an Electrically Variable Transmission”, thedisclosures are each hereby incorporated herein by reference.

As shown in FIG. 4, the supervisory hybrid electric vehicle controller402 outputs a torque engine T_(e) _(—) _(CMD) command to the engine anda torque clutch command T_(T) _(—) _(CMD) to the clutches. Thesupervisory hybrid electric vehicle controller 402 also outputs torquelimits for both motors T_(a) _(—) _(Lim), T_(b) _(—) _(Lim) and theclutches T_(CL) _(—) _(Lim), a battery power limit P_(BAT) _(—) _(Lim)and a desired output torque T_(o) _(—) _(des) to the constraintsevaluator 406. An optimum engine speed n_(e) _(—) _(opt) is output tothe input speed profiler 404. An actual engine torque T_(e) _(—) _(ACT)is output from the engine to the constraints evaluator 406, feedforwardcontroller 408 and the hybrid transmission plant 420. Actual first andsecond clutch torques T_(CB) _(—) _(ACT), T_(C2) _(—) _(ACT) are outputfrom the clutches to the constraints evaluator 406, feedforwardcontroller 408 and the hybrid transmission plant 420.

The constraints evaluator 406 additionally inputs an input (i.e.,engine) speed profile n_(i) _(—) _(prof) and an input accelerationprofile {dot over (n)}_(i) _(—) _(prof) from the input speed profiler404, and the output speed n_(o), output acceleration {dot over (n)}_(o)and input acceleration {dot over (n)}_(i) from the hybrid transmissionplant 420. The constraints evaluator 406 outputs the maximum and minimuminput accelerations {dot over (n)}₁ _(—) _(max), {dot over (n)}₁ _(—)_(min) to the input speed profiler 404, and the requested output torqueT_(o) _(—) _(req) to the feedforward controller 408. The output speedn_(o), output acceleration {dot over (n)}_(o) and input acceleration{dot over (n)}_(i) from the hybrid transmission plant 420 are also inputby the feedforward controller 408. The feedforward controller 408additionally inputs the input speed profile n_(i) _(—) _(prof) and theinput acceleration profile {dot over (n)}_(i) _(—) _(prof) from theinput speed profiler 404. The feedforward controller 408 outputsfeedforward torques for the first and second electric motors T_(aFF),T_(bFF) to respective summation blocks S2, S3.

The input speed profile n_(i) _(—) _(prof) from the input speed profiler404 is also output to a subtraction block S1, which also receives theinput speed n_(i) from the hybrid transmission plant 420. The differencebetween the input speed n_(i) and the input speed profile n_(i) _(—)_(prof) is input by the feedback controller 410. The feedback controller410 outputs feedback torques for the first and second electric motorsT_(aFB), T_(bFB) to the summation blocks S2, S3, respectively. The firstmotor controller 412 inputs the summation of the feedforward first motortorque T_(aFF) and the feedback first motor torque T_(aFB). The secondmotor controller 414 inputs the summation of the feedforward secondmotor torque T_(bFF) and the feedback second motor torque T_(bFB).

The actual first motor torque T_(aACT) and actual second motor torqueT_(bACT) are input by the hybrid transmission plant 420, which alsoinputs a load torque T_(Load). The hybrid transmission plant 420 alsooutputs the actual output torque T_(o) _(—) _(act), clutch torqueT_(CL), battery power P_(BAT) and other parameters used by thepowertrain 400.

As noted above, a vehicle with the disclosed powertrain 400 can operatein many modes depending on the states of the first and second clutchesCB₁, C₂. In the IB-EV mode, the engine is held stationary by the “brake”clutch CB₁ and the second clutch C₂. Only the high-voltage batteryprovides the motive power to propel the vehicle using the two electricmotors EMA and EMB (i.e., electric vehicle or EV drive). The under drivemodes enable a higher ratio between the engine and the main planetarygear carrier and provide more output torque for lower engine torque. Theover drive modes enable a lower ratio between the engine and the mainplanetary gear carrier. This mode moves engine operation to a highertorque lower RPM condition for the same engine power.

During the EV modes illustrated in FIGS. 2 and 3, when more motive poweris needed to accelerate the vehicle or initiate travel at higher speeds,the engine has to be fired to generate additional power to meet thedriver's demand (i.e., initiating a hybrid drive). The transitionalperiod from the EV drive mode to the hybrid drive mode using thecombustion engine and the power from the battery/electric motors isreferred to as “engine starts.”

Engine starts can be performed when switching from EV drive to eitherthe UD or OD with engine on (EO) modes. One of the clutches CB₁ (for IBto OD) or C₂ (for IB to UD) needs to be disengaged prior to starting theengine. For instance, for the IB to UD transition, when the engine startcontrol system receives the request to crank up the engine, it firstrelays the request to disengage the second clutch C₂ in a controlledmanner. At the same time, the pressure on the first clutch CB₁ ismaintained so that the first clutch CB₁ remains fully engaged.

The objective of engine starts from IB to UD can be described asfollows: use electric motor torques to generate input acceleration {dotover (n)}_(i) and speed up the engine, at the same time, the electricmotor torques are coordinated to meet the driver's torque request T_(o)_(—) _(req). During the process, engine torque T_(e) and slipping clutchtorque T_(C2) are treated as known disturbances. The feedforward motortorques T_(aFF) and T_(bFF) for the engine start control can bedetermined by the feedforward controller 408 as follows:

$\begin{matrix}{\begin{bmatrix}T_{aFF} \\T_{bFF} \\T_{{CB}\; 1}\end{bmatrix} = {{\begin{bmatrix}* \\* \\*\end{bmatrix} \cdot T_{o\_ req}} + {\begin{bmatrix}* & * \\* & * \\* & *\end{bmatrix} \cdot \begin{bmatrix}T_{e\_ ACT} \\T_{C2{\_ ACT}}\end{bmatrix}} + {\begin{bmatrix}* & * & * & * \\* & * & * & * \\* & * & * & *\end{bmatrix} \cdot \begin{bmatrix}{\overset{.}{n}}_{i\_ prof} \\{\overset{.}{n}}_{o} \\n_{i\_ prof} \\n_{o}\end{bmatrix}}}} & (1)\end{matrix}$

{dot over (n)}_(i) _(—) _(prof) and n_(i) _(—) _(prof) are the desiredengine acceleration and speed during starts. The shape of the desiredengine speed can be tailored for different engine starting types, suchas smooth starts or aggressive starts (in the input speed profiler 404using e.g., n_(e) _(—) _(opt)). Furthermore, the peak accelerationduring engine start events has to be restricted within motor/clutchtorque limits and battery power limits as shown below:

$\begin{matrix}{\begin{bmatrix}T_{aFF} \\T_{bFF} \\T_{{CB}\; 1}\end{bmatrix} = \left. {{\begin{bmatrix}* \\* \\*\end{bmatrix} \cdot {\overset{.}{n}}_{i\_ lim}} + {\begin{bmatrix}* & * & * \\* & * & * \\* & * & *\end{bmatrix} \cdot \begin{bmatrix}T_{o} \\T_{e\_ ACT} \\T_{C\; 2{\_ ACT}}\end{bmatrix}} + {\begin{bmatrix}* & * & * \\* & * & * \\* & * & *\end{bmatrix} \cdot \begin{bmatrix}{\overset{.}{n}}_{o} \\n_{o} \\n_{i}\end{bmatrix}}}\Rightarrow\left\{ \begin{matrix}{\overset{.}{n}}_{i\_ max} \\{\overset{.}{n}}_{i\_ min}\end{matrix} \right. \right.} & (2)\end{matrix}$

T_(CB1) is the reaction torque of the engaged grounding clutch CB₁.Unlike the feedforward torques T_(aFF) and T_(bFF), T_(CB1) is notactively adjusted during the starting process, but the torque limit ofCB₁ (determined by the clamping pressure, disc dimension and frictionmaterial) will impose constraints on the magnitude of peak inputacceleration and admissible motor torques. On the right hand side of theabove equation, all terms can be measured or estimated except T_(o) _(—)_(req). However, given torque limits T_(a) _(min) , T_(b) _(min) , T_(b)_(max) , T_(CB1) _(min) , T_(CB1) _(max) , as well as battery powerlimits P_(BAT) _(min) and P_(BAT) _(max) , the constraints on T_(o) _(—)_(req) can be determined. Moreover, during short-duration events such asengine start, the second electric motor's torque limit T_(b) _(max) canbe raised using a boost of a higher current/voltage. In the end, thefeedforward motor torques for engine-start control T_(aFF) and T_(bFF)can be obtained since all the right hand terms are available.

Due to model inaccuracy and uncertainties, however, the feedforwardcontrol alone cannot guarantee robust tracking of desired engine speed.The feedback controller 410 implemented as proportional-integral (“PI”)generates complementary motor torque commands T_(aFB), T_(bFB) based onthe deviation of actual engine speed from the desired one:

$\begin{matrix}{\begin{bmatrix}T_{aFB} \\T_{bFB}\end{bmatrix} = {{PI}\left( {n_{i\_ prof} - n_{i}} \right)}} & (3)\end{matrix}$

The final motor torque commands are combined and sent to the twoelectric motor controllers 412, 414. The actual motor torques, alongwith the actual engine torque and the off-going clutch torque, are theinputs to the physical transaxle system. The input torques overcome theload torque and generate accelerations to the input and output shafts.FIGS. 5 and 6 illustrate an example of vehicle responses and majorvariable traces during a drive cycle.

The above-described engine start control system and method achieve acoordinated and robust control of the electric motor torques and clutchtorques, which enable the transition from electric vehicle drive tohybrid drive while also meeting output torque request and actuatorconstraints. The system and method disclosed herein enable thetransition of the engine speed from 0 RPM (in the IB-EV mode) to adesired level in under drive or over drive modes using the coordinatedcontrol of the electric motor torques and clutch torques. The system andmethod described herein ensures smooth and customizable engine startingquality and favorable drivability during the transition.

What is claimed is:
 1. An engine start control system for a vehiclehaving an electrically variable transmission, said control systemcomprising: a supervisory controller adapted to input vehicle operatingconditions and driver inputs, said supervisory controller adapted todetermine and output a plurality of torque limits, a desired outputtorque, and an optimum engine speed; an input speed profiler adapted togenerate and output an input speed profile and an input accelerationprofile based on the optimum engine speed; a constraints evaluatoradapted to generate and output a requested output torque based on theplurality of torque limits and desired output torque; a feedforwardcontroller adapted to generate first and second feedforward motortorques based on the requested output torque and the input speed andinput acceleration profiles; a feedback controller adapted to generatefirst and second feedback motor torques based on a difference betweenthe input speed profile and an actual input speed; a first electricmotor controller that receives a summation of the first feedforward andfeedback torques and generates an actual first motor torque used tocontrol the first electric motor when the engine is being turned on oroff; and a second electric motor controller that receives a summation ofthe second feedforward and feedback torques and generates an actualsecond motor torque used to control the second electric motor when theengine is being turned on or off.
 2. The control system of claim 1,wherein the driver inputs comprise at least a throttle position.
 3. Thecontrol system of claim 1, wherein the constraints evaluator is furtheradapted to output a minimum input acceleration threshold and a maximuminput acceleration threshold and said input profiler uses the minimumand maximum input acceleration thresholds when generating the inputspeed profile and the input acceleration profile.
 4. The control systemof claim 1, wherein the supervisory controller is further adapted tooutput an engine torque command to the vehicle engine.
 5. The controlsystem of claim 4, wherein the engine is adapted to output an actualengine torque to the constraints evaluator and the feedforwardcontroller, the constraints evaluator uses the actual engine torque whengenerating the requested output torque, and the feedforward controlleruses the actual engine torque when generating the first and secondfeedforward motor torques.
 6. The control system of claim 1, wherein thesupervisory controller is further adapted to output a clutch torquecommand to first and second clutches of the transmission.
 7. The controlsystem of claim 6, wherein the first and second clutches arerespectively adapted to output first and second actual clutch torques tothe constraints evaluator and the feedforward controller, theconstraints evaluator uses the first and second actual clutch torqueswhen generating the requested output torque, and the feedforwardcontroller uses the first and second actual clutch torques whengenerating the first and second feedforward motor torques.
 8. Thecontrol system of claim 1, wherein the plurality of torque limitscomprises a first electric motor torque limit, a second electric motortorque limit, a clutch torque limit, and a battery power limit.
 9. Thecontrol system of claim 1, further comprising a hybrid transmissionplant adapted to input the actual first and second motor torques, a loadtorque, actual engine torque and actual first and second clutch torques,said hybrid transmission plant being adapted to output an actual outputspeed and acceleration and an actual input speed and acceleration. 10.The control system of claim 9, wherein the constraints evaluator usesthe actual output speed and acceleration and the actual inputacceleration when generating the requested output torque, and thefeedforward controller uses the actual output speed and acceleration andthe actual input acceleration when generating the first and secondfeedforward motor torques.
 11. A method of controlling first and secondelectric motors of a vehicle having an electrically variabletransmission during an engine start/stop operation, said methodcomprising using a processor to: determining an input speed profile andan input acceleration profile based on an optimum engine speed;determine a requested output torque based on a plurality of torquelimits and a desired output torque; determine first and secondfeedforward motor torques based on a requested output torque and theinput speed and input acceleration profiles; determine first and secondfeedback motor torques based on a difference between the input speedprofile and an actual input speed; sum the first feedforward andfeedback motor torques and generate an actual first motor torque; sumthe second feedforward and feedback motor torques and generate an actualsecond motor torque; and using the actual first and second motor torquesto respectively control the operation of the first and second electricmotors when the engine is being turned on or off.
 12. The method ofclaim 11, wherein the optimum engine speed, torque limits and desiredoutput torque are determined using vehicle operating conditions and avehicle throttle position.
 13. The method of claim 11, furthercomprising: generating a minimum input acceleration threshold;generating a maximum input acceleration threshold; and ensuring that theinput acceleration profile is constrained with the minimum and maximuminput acceleration thresholds.
 14. The method of claim 11, furthercomprising determining an actual engine torque and using the actualengine torque when determining the requested output torque and the firstand second feedforward motor torques.
 15. The method of claim 11,further comprising: determining a clutch torque command; and outputtingthe determined clutch torque command to first and second clutches of thetransmission.
 16. The method of claim 11, wherein the plurality oftorque limits comprises a first electric motor torque limit, a secondelectric motor torque limit, a clutch torque limit, and a battery powerlimit.
 17. The method of claim 11, further comprising determining anactual output speed and acceleration and an actual input speed andacceleration based on the actual first and second motor torques, a loadtorque, actual engine torque and actual first and second clutch torques.